The embodiments described herein relate generally to a trace detection technique for chemical substances, and, more particularly, to using additives with isotopic patterns to detect contraband substances such as explosives, narcotics, pesticides, and chemical warfare agents by means of high resolution mass spectrometry or ion mobility spectrometry.
In many known substance detection techniques, adduct ions are commonly used. Adducts are products of combining two or more distinct molecules to form a single reaction product containing all of the atoms of all of the component molecules, thereby forming a distinct molecular species. Such distinct molecules may be positive or negative ions and the associated adduct ions are formed in either a positive ion mode or a negative ion mode, respectively, to enhance the sensitivity of spectrometry devices for certain classes of compounds of interest. The use of adducts facilitates ionization through ion attachment to only slightly ionizable or completely non-ionizable substances. Also, the use of adducts facilitates ionization through stabilizing fragile molecular ions which otherwise would fall apart during analysis thereby producing multiple fragments of substances of interest that may decrease the sensitivity to their detection. At least some of the known substance detection techniques include using such adducts as additives with dopants in both ion mobility spectrometry and mass spectrometry. Typical dopants used in trace detection of explosives are chlorinated compounds, e.g., in a negative ion mode, dichloromethane and chloroform, and, in positive ion modes, ammonia-based compounds, e.g., ammonia carbonate and anhydrous ammonia.
Some halogenated compounds such as chlorine and bromine have distinct natural isotopic patterns that are very useful for substance analysis. The natural distribution of isotopes, i.e., natural isotopic patterns for chlorine, e.g., 35Cl and 37Cl, a difference of two neutrons, is about 3:1 and for bromine, i.e., 79Br and 81Br, a difference of two neutrons, is about 1:1. The well-known rules of mass spectra interpretation facilitate using the associated isotopic patterns to determine the number of carbon, chlorine, and bromine atoms in a molecule of interest. Since the stable isotopes of common elements differ only by one or two neutrons, only high resolution mass spectrometry or ion mobility techniques may be appropriate for analysis of such isotopic patterns.
Known tandem mass spectrometry systems and methods currently play an almost exclusive role in the identification of chemical compounds of interest by means of mass spectrometry. In known tandem mass spectrometry methods, ions are typically fragmented by collisions with buffer gas molecules, and the fragmentation pattern is compared with the predetermined or calculated database of fragment ion masses. Such known tandem mass spectrometry methods are fairly successful in identifying substances of interest. However, such known tandem mass spectrometry systems are large, heavy, power intensive, and expensive. In addition, those trace detection systems that are the most attractive candidates due to size, weight, and power restrictions, e.g., single quadrupole mass spectrometry devices are not able to use tandem mass spectrometry methods for unambiguous identification of chemical substances.